Apache ignoring PHP headers when sending a 304

Question

When I set a custom header in Apache + mod_php5, this works fine:

header(\'Foo: Bar\');

But when I try this while also sending a 3

Answer 1

Does this not answer the question?

If the conditional GET used a strong cache validator (see section 13.3.3), the response SHOULD NOT include other entity-headers. Otherwise (i.e., the conditional GET used a weak validator), the response MUST NOT include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers.

from http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5